It is common practice in high voltage circuits to use fuses which contain arc extinguishing materials. Thus, when the fusible element (usually a metal wire) melts due to the passage of an overload current, the metal wire is fragmented and the metal particles cause arcing within the fuse casing. Arcing can be both dangerous and damaging to the circuit. The arc extinguishing materials in the fuse serve to cool and extinguish the arc.
Although a variety of arc extinguishing materials have been proposed and used with some measure of success, in practice there are several drawbacks and disadvantages associated with these fuses. One such drawback is the breakdown of the fusible wire element. This is because the fusible wire element is subjected to frequent thermal expansions and contractions resulting from variations in current loads in the circuit. Since the ends of this fusible element are securely fixed to the terminals of the fuse, the thermal expansion and contraction of the fuse produces stresses and strains within the fusible wire element which eventually causes a breakdown of the wire due to so-called elasticity fatigue. Even though the foregoing phenomena are experienced regardless of whether arc extinguishing materials are used or not, the use of arc extinguishing materials aggravate the problem. This is because the fusible wire element is usually densely surrounded by the arc extinguishing materials and hence the wire is prevented from movement and cannot relieve the stresses and strains produced in its structure.
The use of arc extinguishing materials involve other disadvantages. For example, the surface of the fusible wire element can be damaged due to abrasion caused by continued contact with the moving particles of the arc extinguishing materials which are usually in granular or pulverized forms. Also, the movements of the arc extinguishing particulate matters in the casing due to the aforementioned thermal cycles result in localized changes in volume and contact points from time to time, thus resulting in unstable fusing characteristics.
The use of arc extinguishing materials may even be detrimental in some instances, such as when the overcurrent is not sufficiently strong, i.e., it is lower than a certain limit, and thus a so-called "insulating distance" is not attained. By "insulating distance" is meant a wide gap between the melted tips of the fusible wire element so that no further arcing takes place and the current is thus interrupted. The metal particles generated due to melting of the fusible wire element are trapped by the arc extinguishing materials, are wafted near the fusible wire element and cause repeated arc generation. Such prolonged arcing tends to overheat and thus melt the arc extinguishing materials therefore reducing the insulating capacity of the arc extinguisher and often results in failure of interruption.
It is therefore an object of this invention to provide a fuse which is free from the aforementioned drawbacks and limitations.
It is a further object of this invention to provide a fuse which does not contain arc extinguishing materials.
It is still another object of this invention to provide a fuse which does not require the use of arc extinguishing materials due to its unique construction and the novel arrangements of its various components.
It is yet a further object of this invention to provide a fuse having a novel construction and excellent fusing and current interruption characteristics.
The foregoing and other features and advantages of this invention will be more fully comprehended from the following detailed description and the accompanying drawings.